This invention relates to downhole separators used in oil and gas wells, and in particular, to an orbital downhole separator driven by an internal motor and having a flow conditioner to improve fluid separation and control systems for such separators.
Oil and/or gas wells quite often pass through a productive strata the yield of which includes oil, gas and other valuable products but also includes undesirable and unwanted constituents such as salt water. In oil well production operations, relatively large quantities of water are frequently produced along with the valuable petroleum products. This is particularly true during the latter stages of the producing life of a well. Bringing this water to the surface and handling it there represents a significant expense in lifting, separation and disposal.
Various methods have been employed for extracting the valuable petroleum yield from the unwanted constituents. Some have involved the pumping of the total yield of the well to the surface and then using various methods for separating the valuable products from the unwanted portion. In addition, the unwanted portion of the yield, after having been pumped to the well surface and separated, often has been pumped downwardly again through a remote wellbore into a disposal layer. This, of course, also increases expenses.
In some oil wells, the unwanted constituents can amount to as much as 80% to 90% of the total formation yield. Accordingly, to obtain a given volume of valuable petroleum from the well fluid, eight or nine times the volume of the petroleum must first be pumped to the surface and then separated from the unwanted portion. As already noted, this process can be very slow and expensive. Although the problem of producing substantially water-free oil from the well reservoir may occur at any state in the life of an oil well, the proportion of water to valuable yield generally increases with time as the oil reserves decline. Ultimately, when the lifting cost of the combined petroleum and water constituents exceeds the value of the recovered oil, abandonment of the well becomes the only reasonable alternative.
Many procedures have been tried for producing water-free oil from a formation that has a large quantity of water. For example, the oil and water produced are pumped or otherwise flowed together to the surface where they are treated to separate the petroleum from the water. Since the volume of water is usually much greater than that of the oil, the separator must handle large volumes of fluid and therefore is correspondingly large and expensive. Moreover, the water produced contains mineral salts which are extremely corrosive, particularly in the presence of air. Also, flowing the oil and water together upwardly through the well sometimes forms emulsions that are difficult to break. Such emulsions frequently must be heated in order to separate them even when in the presence of emulsion-treating chemicals. The heating of the large amount of water, as well as the small amount of oil requires an expenditure of large amounts of energy, reducing the net equivalent energy production from the well.
Water produced from deep formations within the earth frequently contains large amounts of natural salts. For this reason, the salt water brought to the surface cannot be disposed of by allowing it to flow into surface drains or waterways. Relatively small amounts of salt water can sometimes be disposed of by draining into a slush pit or evaporation tank. The normally required disposal method for large volumes of salt water, however, is to introduce the water into a subsurface formation. This requires a disposal well for receiving the produced salt water.
By returning the water to the same formation in this manner, the water is disposed of and also acts as a re-pressurizing medium or drive to aid in maintaining the bottomhole pressure for driving the well fluids toward the producing well. But, in those areas where producing wells are widely separated, the cost of drilling disposal wells for each producing well is often prohibitive. In such instances, it is necessary to lay a costly pipeline-gathering network to bring all of the produced water to a central location, or alternatively, to transport the produced water by trucks or similar vehicles. Regardless of the method for transporting the waste salt water from a producing well to a disposal well, the cost of the disposal can be, and usually is, prohibitive. Furthermore, fluids from subterranean reservoirs can have undesirable characteristics such as creating excessive pressure and super-heating of the fluids. If excessive pressure is present, then surface equipment, such as a choke manifold, must be installed to choke the flow pressure down to about 2,000 psi, a manageable pressure. If a highly pressurized fluid depressurizes within a short period of time, then a large portion of the gas is “flashed”. This reaction adversely affects the desirable petroleum from the formation yield. In general, both well seals and surface equipment suffer in the presence of excessive fluid pressure and heat. This equipment is expensive in terms of maintenance and capital costs. Thus, it is highly desirable to minimize these undesirable characteristics of the well flow before being brought to the surface.
Downhole separation of water from oil in a well is a desirable approach for disposal of formation water in the well. It eliminates or reduces the excessive costs discussed above required to pump the water to the surface and dispose of it. Furthermore, the greatly reduced environmental impact of the produced water is another factor in making this approach attractive.
Earlier downhole separators are shown in U.S. Pat. Nos. 5,156,586; 5,484,383; and 6,367,547.
The use of downhole separators eliminates or reduces the excessive costs discussed above to pump the water and dispose of it. Furthermore, the greatly reduced environmental impact of the produced water is another factor in making this approach attractive.
Improvements of prior art separators are desirable to further improve efficiency. The present invention includes a separator with a rotating cylinder and a variety of flow conditioners to increase the efficiency of the separator. One embodiment of the present invention adds an impeller to pump the fluid into an annulus to increase tangential fluid velocities. In another, a stator is used to orient the fluid to enter the impeller with a minimum of shearing action. In still another, baffles are positioned in an annular space in the rotor to force the fluid to rotate at the shaft velocity which will improve the separation efficiency.
In another embodiment, a multi-lip cup designed to facilitate multi-density substances so that they are separated into different conduits is used.
In another embodiment, a smart controller is used to control the speed of the motor to modulate the oil concentration in the outlet water. This control function is achieved without the use of a sensor for oil-concentration feedback by measuring the voltage and the current of the motor. The voltage is a measure of the rotor speed, and the current is a function of the applied torque. The torque in turn varies with the water-cut (the ratio of water to oil). By establishing the relationship between the torque and the water-cut and the speed, the motor speed can be adjusted to operate at the desired set point.
A further embodiment utilizes a speed control which has an oil-in-water concentration sensor feedback in conjunction with a conventional PID controller or an adaptive controller for the control function. The motor speed is adjusted to achieve the oil concentration in the out fluid stream on the water side. One way of doing this includes using a valve on the downstream side of the water side which is modulated to achieve the quality of the water to be re-injected. A conventional controller is used to regulate the valve in response to the operating conditions to obtain a desired set-point of the oil content in the re-injection water. An adaptive controller can also be used to control the speed of the motor or the position of the valve using an adaptive algorithm for the controller to drive the concentration of the oil to the desired value.